Eiman Azim - US grants

DESCRIPTION (provided by applicant): Clarifying how neural circuits shape forelimb behaviors can provide insight into motor dysfunction caused by disease or injury, and can potentially improve diagnosis and treatment. The precision of skilled forelimb movements is thought to depend on the conveyance of internal copies of motor commands to cerebellar circuits that refine motor behavior. The inability to access internal copy pathways selectively, however, has made it difficult to assess their function. The goal of this proposal is to evaluate how forelimb behavior is controlled by a set of cervical propriospinal neurons (PNs) that have a simple anatomical means by which to convey copies of pre-motor signals internally; PNs receive descending motor command input, and send bifurcating axonal output to forelimb motor neurons as well as to the lateral reticular nucleus (LRN), a pre-cerebellar relay. These dual projections raise the issue of whether information relayed by the PN internal copy branch regulates forelimb movement. We took advantage of the genetic tractability of mice to: i) ablate PNs, uncovering a selective disruption of reaching behavior; and ii) manipulate PN axonal input to the LRN selectively, revealing a rapid cerebellar-motor feedback loop. Based on these observations, we hypothesize that PN internal feedback circuits contribute to the on-line correction of motor output during reaching. In this proposal, I aim to address three central questions about the organization and function of the PN circuit. During the K99 phase of the award, I will identify which aspects of forelimb movement recruit this feedback pathway by characterizing the dynamics of PN-LRN circuit activity during behavior (Aim 1). To enable assessment of the role of PN feedback, I will develop viral tools to inhibit the PN-LRN circuit, and behavioral approaches to introduce precisely timed perturbations of the limb (Aim 2; K99). With these methods in hand, during the R00 phase I will silence PN output during imposed limb perturbation to investigate the contribution of PN feedback to on-line reaching correction (Aim 2; R00). Finally, I will characterize the supraspinal circuits that are recruited by PN feedback durin reaching correction (Aim 3). Together, these studies will help clarify how cerebellar feedback pathways establish motor precision. The training plan, under the primary mentorship of Dr. Thomas Jessell at Columbia University, provides a comprehensive strategy for acquiring the necessary experimental and professional skills within an exemplary and collaborative neuroscience environment. An experienced team of mentors and collaborators will provide training in skills critical for my short- and long-term success, including: in vivo imaging of neurl activity, acute silencing of synaptic output, electrophysiological mapping of neural circuits, and rigorous design of forelimb behavioral assays. Focused mentor guidance, alongside frequent data presentation and formal and informal instruction, will provide the communication and leadership skills vital for my transition to independence. In the long-term, this support will equi me to lead a laboratory that merges molecular and systems approaches to explore the neural basis of skilled movement.

Project Summary Behavior is movement, and the effective and efficient execution of movement has served as a fundamental evolutionary force shaping the form and function of the nervous system. Control of the forelimbs to interact with the world is one of the most essential achievements of the mammalian motor system, yet unfortunately these behaviors are particularly vulnerable to disease and injury. The execution of skilled limb movements requires the continuous refinement of motor output across dozens of muscles, suggesting the existence of feedback pathways that enable rapid adjustments. The temporal delays of peripheral pathways, however, suggest that sensory feedback alone cannot explain the sophistication of online motor control. In principle, a more rapid source of feedback would be to convey copies of motor commands internally to the cerebellum to generate predictions of motor outcome, reducing dependence on delayed sensory information. Yet putative copy circuits have been difficult to isolate experimentally, leaving their contributions to movement unclear. Mouse genetic tools offer a means to explore a diverse class of spinal interneurons as a neural substrate for internal copies. Cervical propriospinal neurons (PNs) receive descending motor command input and extend bifurcating axons; one branch projects to forelimb motor neurons and the other projects to the lateral reticular nucleus (LRN), a major cerebellar input, providing an anatomically straightforward means to convey motor copies internally. Yet how diverse classes of PN-LRN circuits are organized and how they each contribute to distinct elements of limb behavior remain unclear. Complicating the problem, the field lacks robust ways for deconstructing complex limb movements into component elements (e.g. reaching, grasping, postural control), and objective means for quantifying these behaviors in mice. Hypothesis: Discrete classes of PN circuits convey distinct types of spinal motor copy information to the LRN, each necessary for separate aspects of forelimb control; this functional logic can be resolved with more quantitative, high-resolution and standardized behavioral assays. To test this overarching hypothesis, Aim 1 uses molecular-genetic circuit mapping approaches and single-cell RNA- sequencing to define the anatomical and molecular organization of four classes of PN-LRN circuits. Identifying the fine-grained structure of these diverse pathways will be essential for establishing how internal copies are conveyed to the cerebellum to control forelimb behavior. Aim 2 addresses the need for more sensitive and unbiased behavioral tools by developing novel assays of discrete elements of forelimb behavior and machine learning approaches for automated quantification of forelimb kinematics. Finally, Aim 3 merges these novel behavioral approaches with intersectional genetic tools, electrophysiological recording and circuit-specific perturbation to functionally dissect PN-LRN circuits and define their modular contributions to dexterous limb control. Ultimately, these studies will yield insight into the function of internal copy circuits throughout the nervous system, and help to lay the foundation for better diagnosis and treatment of motor deficits.